The term “porosity” refers to pore space in a material. It can be defined as the fraction of the bulk volume that is occupied by pores or by void space. The individual pores may vary greatly in size and shape within a given solid, and between a given solid and another solid. The width of the pores is commonly assumed to be the diameter of a cylindrical pore, or the distance between the sides of a slit or narrow-shaped pore.
The International Union of Pure and Applied Chemistry issued in 1991 (PURE & APPL. CHEM., Vol 63, N 9, pp 1227-1246, 1991) provides the following recommendations to classify pores according to their size:                Macropores—widths exceeding about 50 nm (nanometers)        Mesopores—widths between 2 and 50 nm        Micropores—smaller than 2 nm        
Mercury intrusion appears to be the most popular method to assess the pore distribution in the meso- and macro-region of pore widths whereas physical adsorption is the main method to measure the micro pores. The common principles of porous solid creation include phenomena such as aggregation and agglomeration; re-crystallization; subtraction and addition. For example, porous glasses are prepared by leaching non-porous templates. The porous structure of zeolites, aluminas and silicas can vary with conditions of crystallization and spray drying. Sintering can be also used to change the pore volume of alumina but the BET surface area decreases in such an operation.
The preparation of carbon molecular sieves is an example of pore structure altering by addition. Treating source particles with hydrocarbons at suitable conditions leads to carbon film deposits at the pore mouths causing narrowing the pores.
There are numerous patents related to the pore system of alumina based supports and catalysts. Some of them are listed as follows: U.S. Pat. Nos. 4,001,144; 4,140,773; 4,179,411; 4,301,037; 4,548,709; 5,260,241; 6,403,256; 6,589,908; and 6,984,310.
In these patents, the pore structure of alumina is created or altered by means such as selection of crystallization conditions, presence of seeds, specific extrusion conditions, additives and etc. The paper “Preparation of Bimodal Aluminas and Molybdena/Alumina Extrudates” by R. E. Tischer published in JOURNAL OF CATALYSIS, Vol. 72, pp 255-265, 1981 describes the following methods to produce bimodal pore structure in alumina extrudates: Partial peptization, coextrusion of salt/Boehmite mixtures, and incorporation of combustible fiber such as filter pulp.
The closest prior art to the present invention is described in U.S. Pat. No. 6,403,526 where the alumina is derived from a mixture of Gibbsite (ATH) and active alumina and an additive component is used as well. Another close art is reported in U.S. Pat. No. 4,001,144 where an alumina precursor of chi-rho-eta structure is treated with carbonate or bicarbonate solutions under pressure at about 100° to 160° C. However, the invention described herein is very different from this prior art.
Acid gases are present as impurities in numerous industrial fluids, i.e., liquid and gas streams. These acid gases include hydrogen halides such as HCl, HF, HBr, HI and mixtures thereof. Hydrogen chloride is a problem in particular. Usually, HCl is removed at ambient temperature with alkali metals modified alumina or metal oxide (mostly ZnO) sorbents. On the other hand, high temperature chloride scavengers are needed for some industrial applications such as the production of hydrogen by steam reforming of hydrocarbons. In these applications, the hydrocarbon feed first passes through a hydrodesulfurization (HDS) or hydrogenation stage that converts the organo-chloride contaminants to HCl. Since the HDS process operates at 350° to 400° C., it is advantageous if the next stage of chloride scavenging also occurs at a high temperature.
Use of alumina loaded with alkali metals as an HCl scavenger is the current “state of the art” solution for the purification of hydrocarbon streams at high temperatures. However, the standard zinc oxide based sorbents cannot be applied in such applications because of the volatility of the resulting zinc chloride product.
The existing sorbents for high temperature applications need improvements in terms of chloride loading, reduced reactivity towards the main stream and physical stability in service.
Alumina modified with alkali or alkaline earth elements is known as a good chloride scavenger. Recently, Blachman disclosed in U.S. Pat. No. 6,200,544 an adsorbent for removing HCl from fluid streams comprising activated alumina impregnated with alkali oxide and promoted with phosphates, organic amines or mixtures thereof.
In an attempt to increase the adsorbent performance, U.S. Pat. No. 5,897,845 assigned to ICI claimed absorbent granules comprising an intimate mixture of particles of alumina trihydrate, sodium carbonate or sodium bicarbonate or mixtures thereof and a binder wherein the sodium oxide (Na2O) content is at least 20% by weight calculated on an ignited (900° C.) base. This material was designated for use at temperatures below 150° C.
The present application targets developing alumina particulates with special pore structure suitable for mass-transfer limited applications. A practical and cost effective method to produce such alumina particulates is targeted as well. There are many examples of the positive effect of the presence of large pores in catalysts and adsorbents. Hydrotreatment of petroleum fractions is an appropriate example on the catalyst side while HCl removal from gas and liquid streams illustrates the technical problem to be solved on the adsorbent side. The present application focuses on the latter.
Trace hydrogen chloride contaminates the effluent in major catalytic processes in the hydrocarbon industry such as the UOP processes CCR Reforming and Oleflex. If not removed from the effluent, HCl can cause corrosion and plugging of the equipment and poison sensitive catalysts downstream. Therefore, HCl scavengers are regularly used in the hydrocarbon industry. Alumina modified with alkali, mostly sodium, and alkaline earth, mostly calcium, metals dominates the HCl removal applications. Some other metal oxide or carbonate based materials are also in use.
The plugging of the pore structure with “green oil” produced via side reactions of reactive stream components on the chlorinated scavenger (adsorbent) is a common cause of premature failure of the scavenger. Another cause is the liquid condensation in the pore system especially when two phase flow occurs. In both cases, the efficiency of the material decreases dramatically. Often, replacement with the fresh material only solves the problem.
The special trimodal pore structure provided with this invention best addresses the problems of current industrial HCl scavengers. Moreover, the special pore structure of the alumina is combined with a high concentration of the active component, an alkali metal, which determines the performance potential in HCl removal.
Last, but not least, all this is achieved in a cost effective manner. Generally, HCl in gas or liquid hydrocarbon streams must be removed from such streams to prevent unwanted catalytic reactions and corrosion to process equipment. Furthermore, HCl is considered a hazardous material and the release of HCl to the environment needs to be avoided.
There are currently two main classes of HCl scavengers. The first group comprises the alkali or alkaline-earth doped aluminas. The alkali metal content of these adsorbents calculated as an oxide (Na2O) is typically between 8 and 10%. The scavengers of this group achieve a relatively low Cl loading, typically 7 to 9%. The second group consists of intimate mixtures of alumina, carbonate (bicarbonate) and binder. A typical material from this group is described in U.S. Pat. No. 5,897,845. The Na2O content is at least 20 mass-%, which determines the high potential Cl loading of this material. However, scavengers of this type cannot be used at temperatures higher than 150° C. They have low BET surface area and insufficient porosity to provide high loading and the inability to function at the high temperatures present in certain applications. For example, in the '845 patent, minimum BET surface area is greater than 10 m2/g and one commercial product that is intended for high temperature chloride removal has a BET surface area of about 66 m2/g. Accordingly, there remains a need for improved halide scavengers with high loading capacity that can operate at high temperatures, such as above 150° C.